1. Field of the Invention
This invention relates to helical screw type compressors with axial fluid flow in which means is provided for varying both the internal volume ratio and the capacity.
2. Description of the Prior Art
The U.S. Pat. No. 4,516,914, to Murphy and Spellar, discloses a helical screw type compressor having a slide valve and a slide stop member in which the pressures at the suction and discharge are sensed and corresponding signals are sent to a micro-processor which calculates the system pressure ratio and causes the selective repositioning of the slide valve and the slide stop in accordance with predetermined criteria.
U.S. Pat. No. 3,088,659, to Nilsson et al., discloses a helical compressor having slide valve and slide stop members which can be adjusted for regulating the volume ratio and capacity.
U.S. Pat. No. 3,432,089, to Schibbye, discloses a helical screw compressor having a slidable valve element for adjustment of its capacity.
U.S. Pat. No. 3,549,280, to Linneken, discloses a helical screw compressor having a slide which is connected to a pressure equalization piston which is charged by the exit or inlet pressure of the pump medium in a direction opposite to the charging of the slide.
U.S. Pat. No. 4,362,472, to Axelsson, discloses a helical compressor in which the opening of the outlet port is controlled in order to control the volume ratio.
U.S. Pat. No. 4,388,048, to Shaw et al., discloses a helical screw compressor in which two end-to-end pistons are used to provide stepwise control of capacity. U.S. Pat. No. 4,412,788 Shaw et al., is another in which the capacity is controlled by controlling the movement of a slide valve.
U.S. Pat. No. 4,455,131, to Werner-Larsen, discloses a helical screw compressor in which the valve member is formed in three parts instead of two as in the U.S. Pat. No. 3,088,659, to Nilsson.
U.S. Pat. No. 4,508,491, to Schaefer, discloses a helical screw compressor having a capacity control slide valve and a modular unloading assembly which is integral with the compressor hermetic casing.
British Pat. No. 2,138,971A discloses a helical screw compressor in which the volume ratio is varied by a valve which is operated in response to the ratio of outlet and inlet pressure acting on a control valve.